Was one of the million ways that you tried.....changing the name of the wireless network on the AirPort Extreme and also changing the wireless network password?
Keep the wireless network name simple
Use only letters and numbers.....no special characters (like an apostrophe or asterisk, etc)
Limit of 20 characters or less in the name....8-10 would be ideal.
No blank spaces
Same rules for the wireless network password
If that does not work, I can only conclude that the operating system on the iPhone has become corrupted....and this is an iPhone problem....not an AirPort Extreme problem.
Best to post in the iPhone support area to let the experts over there have a crack at things....but in general you will need to reset the iPhone back to its original default settings, then restore it again from your backup. If that does not work, the iPhone needs to be replaced.